1. Field of the Invention
The present invention relates to a toner and a developer for use in electrophotography. In addition, the present invention also relates to an image forming apparatus using the toner.
2. Discussion of the Background
Electrophotography is generally used for image forming apparatuses such as laser printers and dry electrostatic copiers. In electrophotography, an image is formed as follows:    (1) uniformly charging a photoconductive insulating layer (i.e., a charging process);    (2) partially irradiating the photoconductive insulating layer with light to dissipate charges, to form an electrostatic latent image (i.e., an irradiating process);    (3) adhering a toner, which is typically a colored and charged particulate material, to the electrostatic latent image to form a visible image (i.e., a developing process);    (4) transferring the visible image onto a transfer material such as a transfer paper (i.e., a transferring process); and    (5) permanently fixing the visible image onto the transfer material upon application of heat, pressure, and the like (i.e., a fixing process).
In the fixing process, for example, contact heat fixing methods (such that a toner is heat-melted using a heat roller) and non-contact heat fixing methods (such as oven fixing) are used. Recently, since a demand for energy saving increases, copiers are also required to reduce electrical power consumption in the fixing process, which is occupying the most part of the total electrical power consumption in the copier. The contact heat fixing methods have an advantage over the non-contact heat fixing methods in terms of thermal efficiency, and therefore a toner can be fixed at relatively low temperature, resulting in energy saving and downsizing of the apparatus.
However, the contact heat fixing methods tend to cause an offset problem in that part of a fused toner image is adhered and transferred to the surface of the heat roller, and then part of the toner image is retransferred to the following sheet of a transfer material.
In attempting to prevent occurrence of the offset problem, a technique in which the surface of the heat roller is covered with a material having high releasability such as fluorocarbon resins, and a technique in which a fixing oil, such as a silicon oil, is applied to the surface of the heat roller, have been proposed. However, in the latter case, since a fixing device needs a fixing oil applying system, the fixing device upsizes and the fixing system becomes complicated. As a result, manufacturing costs and operation costs increase, and various troubles are likely to be caused.
Moreover, in attempting to prevent occurrence of the offset problem, a technique in which the temperature of a heat roller is decreased is proposed. However, in this case, a toner cannot be sufficiently melted, i.e., the toner cannot be sufficiently fixed.
In terms of energy saving and downsizing of apparatus, toners are required to have good hot offset resistance and good low temperature fixability. In addition, toners are required to have a property such that a toner does not cause a blocking problem even when the toner is stored at the temperature inside an apparatus (this property is hereinafter referred to as thermostable preservability).
Japanese Patent No. 3142297 discloses a toner including two kinds of binder resins having different softening points. In this case, as the amount of a resin having lower softening point increases, low temperature fixability of the toner improves, but the thermostable preservability and hot offset resistance thereof deteriorate. In addition, as the glass transition temperature of the resin having lower softening point increases, the thermostable preservability of the toner improves, but the toner cannot have a good combination of low temperature fixability and hot offset resistance even if the amount of the resin having lower softening point is increased. Moreover, since the difference between the softening point of the resin having lower softening point and the minimum fixing temperature at which the toner can be fixed is too large, the toner cannot be sharply melted at such a low fixing temperature. Because of these reasons, a need exists for a toner having better low temperature fixability and better hot offset resistance.
Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2004-302458 discloses a toner including a crystalline polyester resin as a binder resin, which is manufactured by the following method: dissolving or dispersing toner constituents including the crystalline polyester resin and a modified polyester resin capable of reacting with a compound having an active hydrogen group, in an organic solvent to prepare a toner constituent mixture liquid; and dispersing the toner constituent mixture liquid in an aqueous medium containing a particulate resin while reacting the modified polyester resin with a cross-linking agent and/or an elongation agent to prepare toner particles. The crystalline polyester resin has a thermal property such that viscosity thereof rapidly decreases at a temperature at which the toner starts to be fixed, due to its crystallinity. For this reason, the toner shows good thermostable preservability until the toner is melted, and the toner has a sharp melting property, and is fixed on a transfer material because the crystalline polyester resin starts to rapidly decrease its viscosity at the melting point. It is described in JP-A 2004-302458 that such a toner has a good combination of thermostable preservability and low temperature fixability.
JP-A 2003-57875 discloses a toner including a binder resin including the following three kinds of resins: a resin (A) having a softening point of from 120 to 170° C. and a glass transition temperature of from 58 to 75°0 C., and including chloroform insoluble components in an amount of from 5 to 50% by weight; a resin (B) having a softening point of from 90 to 120° C. and a glass transition temperature of from 58 to 75° C.; and a crystalline polyester resin (C) having a melting point of from 80 to 140° C., wherein the binder resin includes chloroform insoluble components in an amount of less than 30% by weight. Further, the toner includes a wax (W) having a penetration of not less than 1.5 and a melting point of from 80 to 110° C. These toner constituents are melt-kneaded, and as a result, the crystalline polyester resin (C) serving as a low melting point material is uniformly dispersed in the resins (A) and (B) having different softening points. It is described in JP-A 2003-57875 that such a toner has a good combination of low temperature fixability, hot offset resistance, and thermostable preservability.
JP-A 2004-151709 discloses a toner including (1) a crystalline polymer having a melting point of from 180 to 280° C. and a thermal property such that the heat quantity at an endothermic peak (determined by a differential scanning calorimeter (DSC)) observed at the melting point is from 25 to 150 mJ/mg and (2) an amorphous polyester having a glass transition temperature of from 30 to 80° C., wherein the amorphous polyester includes an amorphous polyester having a weight average molecular weight of from 3,000 to 20,000 and an amorphous polyester having a weight average molecular weight of from 30,000 to 300,000. In such a toner, crystalline portions of the crystalline polymer form a physical cross-linking structure in the amorphous polyester, and on the other hand, amorphous portions of the crystalline polymer and the amorphous polyester are twisted together, resulting in formation of a kind of a network structure in the toner. It is described in JP-A 2004-151709 that the decreasing rate of viscosity of such a toner at high temperatures is low, and therefore a toner having a good combination of low temperature fixability, hot offset resistance, thermostable preservability, and an ability to produce images having good coloring power is provided.
JP-A 2005-164800 discloses a toner including at least a first binder resin (LR), a second binder resin (HR), and a crystalline resin, wherein the toner satisfies the following relationships: 90≦Tlr3≦125, 155≦Thr3≦210, and 95≦Tcr3 ≦150, wherein Tlr3(°C.) represents a temperature at which the first binder resin (LR) has a viscosity of 1×103 (Pa·s), Thr3 (° C.) represents a temperature at which the second binder resin (HR) has a viscosity of 1×103 (Pa·s), and Tcr3(°C.) represents a temperature at which the crystalline resin (CR) has a viscosity of 1×103 (Pa·s). It is described in JP-A 2005-164800 that such a toner can produce high quality images, and has a wide fixable temperature range and good thermostable preservability.
As mentioned above, JP-As 2004-302458, 2003-57875, 2004-151709, and 2005-164800 have disclosed toners including two amorphous polyester resins and a crystalline polyester resin. In these toners, properties of each of the plural resins are maintained in the toner even if these resins have greatly different softening points. However, once these toners are heated, the amorphous polyester resins and the crystalline polyester resin are melted and mixed with each other (i.e., these resins become compatible with each other), and properties of each of the plural resins disappear in the melted toner.
In attempting to solve this problem, JP-A 2004-309517 discloses a toner manufacturing method in which a crystalline resin (100 parts by weight) having a ratio of the softening point to the temperature at which the maximum of the DSC peak representing the melting heat thereof is observed of from 0.6 to 1.3, and a particulate inorganic material (from 0.1 to 10 parts by weight) having a particle diameter of from 20 nm to 3 μm are melt-kneaded. It is described in JP-A2004-309517 that the crystalline resin can easily crystallize by being melt-kneaded together with the particulate inorganic material having a predetermined particulate diameter, thereby providing a toner having good low temperature fixability. However, chargeability of such a toner is changed due to existence of the particulate inorganic material. In addition, since elastic modulus of the toner is also changed due to existence of the particulate inorganic material, the shapes of the toner particles vary, and therefore a problem such that each of the toner particles has different fixing property is caused.
By the way, recently, two-component developing methods using a two-component developer consisting of a carrier and a toner (hereinafter referred to as a developer) is mainly used when the above-mentioned toners are used, in terms of transferability, reproducibility of halftone images, and stability of developability under various temperature and humidity.
In a developing device using a two-component developer, the developer forms magnetic brushes on a developer bearing member while being held thereon. The magnetic brushes (i.e., developer) are transported to a developing region in which the developer bearing member and a latent image bearing member are facing. In the developing region, the magnetic brushes rub the surface of the latent image bearing member so that a toner included in the developer is supplied to an electrostatic latent image. Thus, the electrostatic latent image is developed with the toner. This method is called a magnetic brush developing method.
In the magnetic brush developing methods, as the average particle diameter of the carrier decreases, reproducibility of latent images increases. As a result, high quality images having good reproducibility of letters and good granularity (which represents a visual smoothness of letters) can be produced. Namely, when a carrier having a small particle diameter is used in the magnetic brush developing method, a latent image can be faithfully reproduced, and therefore a high definition image can be easily produced.
On the other hand, as the average particle diameter of the carrier decreases, a magnetic force per particle of the carrier decreases. As a result, a problem such that the carrier adheres to a non-image portion of the latent image formed on the latent image bearing member tends to be caused. (This phenomenon is hereinafter referred to as carrier adherence.)
The reason for the carrier adherence is considered to be as follows. Typically, a carrier is held on a developing sleeve (i.e., the surface of a developing bearing member) due to its magnetism. Simultaneously, the carrier is elecrostatically influenced by charges of an image bearing member because of having a charge. When the electrostatic force acting on the carrier and the image bearing member overcomes the magnetic force acting on the carrier and the developing sleeve, the carrier moves onto the image bearing member.
When the carrier adherence occurs, the quality of the produced image deteriorates. For example, when carrier particles adhered to an image bearing member are transferred and fixed onto a transfer material (such as a paper), an image having noise is produced. When such carrier particles are not transferred onto a transfer material, an image having white spots is produced.
Carrier adherence is more likely to be caused when the image forming speed is high. As the image forming speed increases, the linear speed of a developing sleeve increases, resulting in increase of a centrifugal force acting on a developer held on the developing sleeve. As a result, magnet brushes (i.e., the developer) give a lager impact to the surface of an image bearing member, and therefore a carrier included in the magnet brushes tend to release therefrom.
Recently, high-speed image forming apparatuses are widely used. However, as mentioned above, if carrier particles release from magnet brushes held on a developing sleeve, the released carrier particles adhere to an image bearing member, resulting in carrier adherence.
The carrier particles adhered on the image bearing member cause a problem such that abnormal images such as white spots are produced. Another problem is also caused such that the carrier particles adhered on the image bearing member damage image forming members arranged around the image bearing member.
In attempting to prevent carrier adherence when a carrier having a small particle diameter is used, JP-A 2000-172078 discloses an image forming method using a developer bearing member containing magnetic poles therein of which a main pole has a magnet force of not less than 100 mT, and a developer including a carrier having a volume average particle diameter of not greater than 50 μm, wherein the process speed thereof is not less than 200 mm/sec. It is described in JP-A2000-172078 that when an image bearing member contains magnetic poles therein of which a main pole has a relatively large magnet force, carrier adherence tends not to occur even in such a high-speed image forming method using a two-component developer including a carrier having a small particle diameter.
However, it is still difficult to completely prevent carrier adherence even in such an image forming method. There is a problem such that a part of a toner on the developer bearing member cannot move to an image bearing member because the developing bearing member has too strong a magnetic force. As a result, fogging is observed on the resultant images. In addition, manufacturing cost of the apparatus increases because of loading magnetic poles therein.
Because of these reasons, a need exists for an image forming apparatus utilizing a two-component developing method which does not produce abnormal images (caused by carrier adherence), and produces high definition and high quality images.